L&W Porolog is used in machines producing the following grades: sack paper, MG-kraft paper, cigarette paper, bank note paper, OTC paper, photo base paper, saturating paper and base paper before coating (fine paper).

L&W Porolog consists of a measuring head and a main cabinet with display. An air hose, with a maximum length of 10 m, connects the cabinet and measuring head This simple construction makes the system easy to install and maintain.

Porosity measurement in base paper before coating

For good results in a coating process, porosity (air permeance), is one important quality parameter that must be checked carefully. If the coating is performed off-line, a certain permeance range for the base paper is specified. If the permeance goes beyond those limits, the paper will cause problems in the coating station or may not be accepted at all by the customer.

For paper machines with on-line coating, the use of on-line permeance measurements are very obvious, since it is more or less impossible to get any laboratory samples of the base paper.

Porosity measurement of sack-kraft paper

For sack paper, good strength in all directions is very important. The porosity is also important. When the sack is filled with for example flour or cement, the air must be able to pass through the paper, or the sack will break.

Porosity measurement of cigarette paper

The buyers of cigarette paper often have strictly defined demands regarding the porosity range of the paper. When smoking a cigarette, air must pass through the paper to dilute and cool the smoke from the burning tobacco, which would be too strong and warm otherwise. Therefore the permeance of the paper will influence the “taste” of the cigarette.

Cigarette paper usually has a grammage of 15–30 g/m2 and a permeance of 30–50 Coresta. Coresta is a unit that is used mainly in the cigarette industry. (30–50 Coresta corresponds to about 25–15 Gurley seconds).

On-line control of the porosity

The results from permeance measurements are most commonly used to control the beating of the pulp in the final refiners. For machine control purposes, on-line measurements are preferred, since these provide results much faster than laboratory tests.

The fast response of an on-line measurement is especially important during the start-up of the machine, or at a grade change, to be able to reach the specified per-meance level as soon as possible.

Factors that may affect the porosity

Following factors may affect the porosity;

• pulp quality
• refining
• broke percentage
• white water circulation
• addition of chemicals
• headbox consistency
• drainage on the wire
• pressing, calendering size press operation

Good control of the permeance will result in less rejects and a better and more even quality of the final product.